1. Field of the Invention
The present invention relates to a method for printing a non-aqueous ink and a printing substrate for a non-aqueous ink that enable roller transfer staining to be suppressed while maintaining high image density.
2. Description of the Related Art
The inkjet printing system is a system in which an inkjet ink having high fluidity is sprayed in the form of ink particles from very fine head nozzles, thereby printing an image onto a printing substrate positioned facing the nozzles, and because high-speed printing can be achieved, particularly in those cases where a line head inkjet printing apparatus containing a plurality of ink heads is used, inkjet printing has become very widespread in recent years.
Inkjet inks can be broadly classified into aqueous inks and non-aqueous inks. Non-aqueous inks are less likely to cause curling or cockling of the printing substrate, thereby facilitating transport of the printing substrate, and are therefore ideal for high-speed printing.
Numerous so-called non-aqueous inks prepared by finely dispersing a pigment within a solvent have been proposed as inks for use within the above type of inkjet printing systems.
For example, the applicant has proposed, in Patent Document 1, a non-aqueous ink comprising a pigment dispersed in a solvent that includes an ester solvent, a higher alcohol solvent and a hydrocarbon solvent. This ink exhibits excellent stability within the printer, and a printed item produced using this ink offers the advantage that, even when laid on top of a printed item printed using a PPC transfer device or a laser printer with the printed surfaces facing each other, the printed images do not stick together.
On the other hand, non-aqueous inks suffer from poor separation of the solvent from the coloring material on the printing substrate, and particularly in those cases where a plain paper is used as the printing substrate, the coloring material and the solvent tend to be prone to penetrating together into gaps between the fibers of the printing substrate, resulting in a reduction in the image density. Here, the term “plain paper” refers to a paper that is not provided with a coating layer.
In response to this problem of reduced image density, a method of obtaining a printed item having high image density has been proposed in which the printed item is produced, using a non-aqueous ink, on a plain paper comprising mainly a pulp and a filler containing mainly a rosette-shaped light calcium carbonate, and also comprising a polymeric compound as a sizing agent, wherein the Stoeckigt sizing degree is within a range from 5 to 80 seconds, and the ash content prescribed in JIS-P8251 is within a range from 20 to 40% (see Patent Document 2).
However, in this method, although the image density increases, the effect of the large ash content of the printing substrate means that the coloring material is more likely to be retained on the surface of the printing substrate. As a result, when the printed item is pressed against a roller, the coloring material may be transferred to the roller, and this transferred coloring material can then be re-transferred to the subsequently transported printed item, thereby soiling the printed item and generating so-called roller transfer staining.    [Patent Document 1] JP 2007-126564 A    [Patent Document 2] JP 2005-193660 A
The present invention has an object of providing a method for printing a non-aqueous ink and a printing substrate for a non-aqueous ink that enable roller transfer staining to be suppressed while maintaining high image density.